The present invention relates generally to fastening systems, and in particular to locking fastener systems and tools that swage or deform a collar to lock it into a fastener.
Various systems for locking threaded fasteners have been developed. Some locking fasteners rely on swaging material from a nut or collar into recesses on a bolt or shear pin. Such a system is disclosed in U.S. Pat. No. 4,601,623 issued to Wallace. The locking fastener of Wallace includes a shear pin or bolt with exterior threading to engage the axial bore of a nut or collar. The nut has a base and an elliptical end portion.
The Wallace system also has an installation tool with an elliptical bore adapted to engage the end portion of the nut. Engaging the nut with the tool and rotating the tool applies torque to thread the nut onto the bolt. Once a predetermined load is reached, the nut is set and the tool rotates further thereby applying radial compression to deform the nut inward toward the bolt. The elliptical bore is larger than the elliptical end portion of the nut. Continued rotation deforms the ends of the elliptical end portion. The deformation of the end portion of the nut continues until the end portion has a generally circular shape and the tool rotates freely on the nut. Deformation of the nut causes it to extend into recesses in the bolt, thereby locking the nut to the bolt.
Another locking fastener system is disclosed in U.S. Pat. No. 4,383,353 issued to Stencel. The locking fastener of Stencel comprises a collar with an interior bore threaded for engagement with a pin. On its exterior, the collar has three lobes that extend radially outward from the collar for engagement with an installation tool. The collar is threaded onto the pin, and at a predetermined load the lobes on the collar deform radially inward to lock the collar and the pin together. The installation tool rotates freely on the collar once the lobes have been deformed.
One drawback to the fastener systems described above and others known in the art is that the preload and locking forces are generated simultaneously. The preload is the tension between the fastener and the collar which places the intervening joint into compression. The locking occurs with the deformation of the collar into the fastener. Since the preload force and locking force are generated at the same time, a portion of the applied torque is used to lock the collar and pin together, and the remaining torque is use to create a preload between the collar and pin. As the lock is formed, it resists increases in the preload. As the lock is formed, less load is available to increase and maintain the preload. Thus, the fasteners known in the art require the application of additional torque to generate the desired preload and overcome the resistance of the lock being formed.
In blind fastener applications, allen wrenches are often inserted into a mating cavity on the bolt to hold the bolt when threading on the collar. These wrenches lock the bolt in a fixed position to prevent the bolt from turning at the same rate as the collar. Because of the force needed to deform the collars to form the lock, the tools known in the art often require a large amount of torque to secure locking fasteners. This large torque must be resisted at least in part by the allen wrenches, which sometimes causes the wrenches to break.
The large amount of torque also gives the installation tool an increased tendency to "cam off" or to be forced away from the nut during fastening. The end portion of the nut resists being deformed and forces the installation tool away from the free end of the nut. This causes the installation tool to become disengaged from the nut and slip as the nut is being tightened.
Typically, the deformable fasteners of the type described above are intended to deform at a predetermined torque which corresponds to a specified preload in the fastener. However, the simultaneous generation of the preload and lock interferes with the accurate setting of the preload and creates variations in the preload between the pin and collar when the fastener is completely installed. Since some of the installation torque is used to lock the fastener together, it is difficult to accurately predict the amount of torque that will be used to generate the preload, and thus, to repeatably predict the amount of preload between the pin and collar.
While the preload and lock may be separated as disclosed in U.S. Pat. No. 4,408,936 to Williamson, the devices and methods know in the art for separating the preload and lock operations make the mass production of such fasteners economically impractical. For example, Williamson discloses a fastener with a collar having a nut section, a drive section, and a shear section. The shear section connects the drive section and the nut section, and fractures when sufficient torque is applied to the drive section. Part of the nut section swages inward, and the drive section torques off. The cost of manufacturing such fasteners is expensive in part because such fastener systems have structural configurations that are difficult to manufacture. Additionally, the fastener of Williamson produces undesired scrap pieces that must be disposed.
As a result there is need for a fastening system and installation tool that generates the preload substantially independent from the lock, allows greater repeatability in the setting of the preload, and reduces the amount of torque required to secure the fastener.